At a preceding stage of the array antenna provided with a plurality of radiation elements, a divider for dividing a high-frequency signal into plural signals and feeding the divided signals to the radiation elements respectively is provided. FIG. 16 shows an array antenna with four radiation elements and a four-way divider. The four-way divider shown in FIG. 16 has one input terminal 1, four output terminals 2 to 5, three Wilkinson-type two-way dividers (see Non-Patent Literature 1) 111, 112, 113, and transmission lines 115 to 120. One radiation element 110 is connected to the output terminals respectively. Respective paths from the input terminal 1 to four output terminals 2 to 5 constitute a tree structure that is formed by the Wilkinson-type two-way dividers 111, 112, 113 and the transmission lines 115 to 120.
FIG. 17 shows a configuration of the Wilkinson-type two-way divider provided to the four-way divider shown in FIG. 16. As shown in FIG. 17, the Wilkinson-type two-way divider is equipped with one input terminal 6, two output terminals 7, 8, transmission lines M101 to M105, and an absorption resistor R100. As shown in FIG. 17, respective paths from the input terminal 6 to two output terminals 7, 8 has a structure that is branched to two paths at the succeeding stage of the transmission line M101.
One branched line of the two paths consists of the transmission lines M102, M104, and the other branched line consists of the transmission lines M103, M105. Electrical lengths of the transmission lines M102, M103 are set to ¼ wave length respectively. Since respective electrical lengths of the transmission lines M102, M103 are set to ¼ wave length, a reflected wave from the output terminals 7, 8 to the input terminal 6 can be reduced and also isolation between the output terminals can be enhanced. The absorption resistor R100 is connected in a position, which is away by ¼ wave length from a branch point toward the output terminal side, to connect two paths. Since the absorption resistor R100 is provided, isolation between the output terminals can be enhanced and also output impedances can be matched.
Assume that a characteristic impedance of the transmission line M101 is Z1, a characteristic impedance of the transmission line M102 is Z2, a characteristic impedance of the transmission line M103 is Z3, a characteristic impedance of the transmission line M104 is Z4, a characteristic impedance of the transmission line M104 is Z5, a resistance value of the absorption resistor R100 is R100, the divided number is N (=2), and a characteristic impedance Z0 of the infinite length line is 50Ω, relational expressions are given as follows:Z1=Z4=Z5=Z0=50ΩZ2=Z3=Z0√{square root over (N)}=70.7ΩR100=Z0·N=100Ω
In the array antenna and the four-way divider explained as above, power levels of respective signals supplied to four radiation elements in phase with each other have an influence upon the radiation characteristic of the array antenna. FIG. 18 shows the radiation characteristic of the array antenna when a power on an equal level is supplied to four radiation elements shown in FIG. 16 respectively. In contrast, FIG. 19 shows the radiation characteristic of the array antenna when a ratio of a level of the power supplied to the radiation elements at both ends out of four radiation elements with respect to a level of the power supplied to two radiation elements in the center is set to 1:4. When a comparison of the level of side lobes and the level of a main lobe is made between FIG. 18 and FIG. 19, the level of side lobes of the radiation characteristic shown in FIG. 19 is lower than that in FIG. 18. In this manner, in order to accomplish the array antenna with the radiation characteristic whose side lobe level is low, the divider that is capable of feeding a power to respective radiation elements at an in-phase and unbalanced power ratio is needed.
Non-Patent Literature 2 discloses a two-way divider that divides an input signal into two signals at an in-phase and any power ratio. Also, Patent Literature 1 discloses a divider in which the two-way dividers for dividing an input signal into two signals at an any power ratio are combined in a multi-stage fashion. In the divider disclosed in Patent Literature 1, the power ratio at the output terminals is set to a desired value in terms of a ratio of the characteristic impedances of the matching lines constituting the two-way divider circuit. Also, since an electrical length difference Δφ of the output terminal of the two-way divider in the n-th stage is adjusted in response to a reflection phase at the branch point in the (n−1)-th stage, a phase error at a center frequency between the output terminals can be reduced and thus the side lobe level of the array antenna can be reduced.
Patent Literature 1: JP-A-5-251910
Non-Patent Literature 1: ERNEST J. WILKINSON, “An N-Way Hybrid Divider”, Vol. MTT-8, IRE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, 1960 January, pp. 116-118
Non-Patent Literature 2: L. I. PARAD, AND R. L. MOYNIHAN, “Split-Tee Divider”, Vol. MTT-13, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, 1965 January, pp. 91-95